“Inside every female, there is a new male in waiting...” This bizarre-looking fish attempts to mate – but there’s one big problem

In this clip from the BBC’s Blue Planet II, filmed in the waters near Sado Island, northern Japan, an extraordinary natural process takes place. A female Asian sheephead wrasse (Bodianus reticulatus), also known as a kobudai, undergoes a dramatic transformation into a male. 

• "In real life Nemo’s dad would become female (and might have even mated with his son) – Discover 10 animals that can amazingly change sex
• The first animal to have sex, the animals that had sex on the moon and penises that can sing: Discover 12 amazing facts you never knew about animal sex

Kobudai are one of numerous species that are sequentially hermaphroditic, meaning individuals of this species are always born female and only change sex when they grow older and reach a critical body size. 

There are no hard-set rules, however – there's no definite age when this occurs, and not every female undergoes the transition. Which makes it all the more complicated for a male who is ready to breed, as he can never know what he’s up against. 

In the video clip, narrated by David Attenborough, a particularly large male (1m long and weighing 15kg) approaches a much smaller female and attempts to mate. But she doesn’t seem to be interested and retreats into the darkness – she will emerge changed.

“Over just a few months, particular enzymes inside her body cease to work and male hormones start to circulate. As time passes, her head expands and her chin gets longer. She has changed into a he,” explains Attenborough. 

• Did sex stop Earth from being boring? Probably, say scientists
• "With sex sessions lasting up to 14 hours, it’s quite a way to bow out" – discover 11 animals that die during or after sex

With this transformation comes a change in temperament. There is only enough space for one male in the territory, and this newly emerged one wants the spot: “The old male who rules all the females here is challenged to a face-off”. 

The fish begin fighting by advancing towards each other with their enormous mouths, colliding over and over in a display of intimidation. Finally, the new male triumphs by injuring the opponent, who retreats into hiding to lick its wounds.

There is a new ruler in the territory. But a new male can’t afford to let his guard down. “Inside every kobudai female, there is a new male in waiting,” concludes Attenborough. 

Top image credit: Martin Voeller/Getty Images